Electrically operated optical shutter

ABSTRACT

An optical shutter comprising a pair of crossed polarizing means, and at least one double-refractive crystal and at least one Z-cut irregular ferroelectric crystal both interposed between the pair of crossed polarizing means, the said crystal having equal retardations, the value of the resulting retardation R of the crystals in the open position of the shutter being 80 &lt; OR = R &lt; OR = 320 or R &gt; OR = 2000, whereby a field at least equal to the magnitude of the coercive field Ec of the irregular ferroelectric crystal or a stress at least equal to the magnitude of the coercive stress Tc of the irregular ferroelectric crystal is applied to the same crystal to rotate the optic axial plane thereof by 90* to pass or stop the natural light transmitted through the shutter as desired.

United States Patent Kurnada et al.

[54] ELECTRICALLY OPERATED OPTICAL SHUTTER [72] Inventors: Akio Kumada,Kodaira; Masashi Koga, Kokubunji, both of Japan [73] Assignee: Hitachi,Ltd., Tokyo, Japan [22] Filed: Mar. 17, 1970 [21] Appl. No.1 20,244

[30] Foreign Application Priority Data Mar. 25, 1969 Japan ..44/22094[52] U.S. Cl ..350/150, 340/173 LS, 340/173.2, 350/149, 350/157, 350/160[51] Int. Cl. ..G02f1/26 [58] Field ofSearch ..350/147, 149,150,157,160-161; 340/173 LT, 173 LS, 1732 [56] References Cited UNITED STATESPATENTS 3,559,185 1/1971 Burns et a1 ..350/157 2,680,146 6/1954Rosenthal ..350/149 OTHER PUBLICATIONS Cross at al., GadoliniumMolybdate A New Type of Ferroelectric Crystal," Phys. Rev, Lett. Vol.21, N0. 2 (Sept. 16,

[ 1 May9,1972

l968)pp. 812-814.

Electronic Engineering, Japanese Discover New OptoelectronicProperties," Vol. 41, (June 1969) p. 6.

Aizu, Possible Species of Ferroelastic Crystals and of SimultaneousllyFerroeletric and Ferroelastic Crystals," J. Physv Soc. Jap. Vol. 27, N0.2 (Aug. 1969) PP. 387- 396.

Aizu et al.; Simultaneous Ferroelectricity and Ferroelasticity of Gd 2(M00 4 )3 .1. Phys. Soc. Jap. Vol. 27, No. 2 (Aug. 1969) p. 51 1.

Primary E.\'aminerDavid Schonberg Assistant Examiner-Paul R. MillerAttorney-Craig, Antonelli & Hill 57 ABSTRACT An optical shuttercomprising a pair of crossed polarizing means, and at least onedouble-refractive crystal and at least one Z-cut irregular ferroelectn'ccrystal both interposed between the pair of crossed polarizing means,the said crystal having equal retardations, the value of the resultingretardation R of the crystals in the open position of the shutter being80 e R 2 320 orR 2000, whereby afield at least equal to the magnitude ofthe coercive field E of the irregular ferroelectric crystal or a stressat least equal to the magnitude of the coercive stress Tc of theirregular ferroelectric crystal is applied to the same crystal to rotatethe optic axial plane thereof by 90 to pass or stop the natural lighttransmitted through the shutter as desired.

12 Claims, 23 Drawing Figures PATENTEDMY 9:912

seam 2 UP 5 INVENTOR S ATTORNEYS PATENTEDMAY 1972 SHEET 3 OF 5 INVENTORS4 KMMHM M/asAs/u mm;

W W V ATTORNEY5 1 ELECTRICALLY OPERATED OPTICAL SHUTTER BACKGROUND OFTHE INVENTION 1. Field of the Invention This invention relates to anoptical shutter device for controlling the transmission or stoppage of apredetermined amount of natural light as desired.

2. Description of the Prior Art Optical shutter devices of the followingtypes are known in the art.

I. The so-called iris stop is used as the optical stop in a photographiccamera. The iris stop is a device for mechanically controlling the crosssection of a light beam, and the control of the cross section of a lightbeam is accomplished mechanically. Such a mechanical control requiresthe device to be operated directly by an electric signal and this leadsto a larger size of the mechanism.

2. An electric eye camera (EE camera) uses a system which utilizes apressure to push the shutter, whereas this system often suffers fromoperational trouble or failure.

3. It is also known to utilize the Kerr effect produced by applying arequired voltage to nitrobenzene or like material so that lighttransmitted through the material may be subjected to a shutter action.The use of the Kerr efi'ect, however, has a disadvantage in that theopening-state of shutter action cannot be kept unless a voltage isapplied.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view forillustrating the correlation between the spontaneous polarization andspontaneous strain produced in an irregular ferroelectric crystal usedin the optical shutter device of the present invention;

FIG. 2a illustrates a hysteresis loop between the electric field appliedto the irregular ferroelectric crystal of the present invention and anpolarization provided thereby;

FIG. 2b illustrates a hysteresis loop between the electric field appliedto an irregular ferroelectric crystal of the present invention and thestrain produced therein;

FIG. 3a illustrates a hysteresis loop between the stress applied to anirregular ferroelectric crystal of the present invention and thepolarization provided thereby;

FIG. 3b illustrates a hysteresis loop between the stress applied to anirregular ferroelectric crystal of the present invention and the strainproduced therein;

FIG. 4 is a view for illustrating the operational principle of theoptical shutter device according to the present invention;

FIGS. 5a and 5b schematically show an example of the optical shutteraccording to the present invention;

FIG. 6 illustrates the relationship between the crystal axis and thespatial axis;

FIGS. 7a, 7b, and 7c are graphs for illustrating the wavelength sensingcharacteristic of an analyzer;

FIGS. 8a, 8b, and 8c and FIG. 9 are graphs for illustrating thetransmitting characteristic of the shutter when it is in the On"position;

FIG. 10 is a CIE color chart showing the retardation R of adouble-refractive crystal plate interposed between two crossed-polars ortwo crossed polarizers;

FIG. 11 is a diagrammatical view of the arrangement according to anembodiment of the present invention;

FIG. 12 is a graph showing the transmission spectrum of the polarizingunit having a retardation R 276 m z;

FIG. 13 is a diagrammatical view of the arrangement according to anotherembodiment of the present invention;

FIGS. 14a and 14b show the arrangement according to yet anotherembodiment of the present invention.

SUMMARY OF THE INVENTION It is an object of the present invention toprovide an electrically operated optical shutter which can effect theshutter action for a transmitted light beam without using any mechanicaldriving means.

It is a further object of the present invention to provide anelectrically operated optical shutter which has a memory effect for adriving voltage signal even after the extinction thereof until thesubsequent application of a voltage pulse of inverse polarity.

It is a still further object of the present invention to provide anelectrically operated optical shutter which effects a shutter action fornatural light.

In order to achieve these objects, the optical shutter of the presentinvention comprises a pair of crossed polarizers disposed in a lightpath, at least one ferroelectric crystal plate havingferroelectric-ferroelastic properties in series with at least onedouble-refractive crystal plate both interposed between the pair ofcrossed polarizers in such a manner that the crystallographic c-axes ofthe crystal plates are normal to each other, wherein a voltage at leastequal to the magnitude of the coercive field of theferroelectric-ferroelastic crystal plate or a stress at least equal tothe magnitude of the coercive stress of the same crystal is applied tothe ferroelectric-ferroelastic crystal plate to thereby rotate the opticaxial plane of the ferroelectric-ferroelastic crystal plate by so as toprovide a shutter action for natural light transmitted through theoptical shutter device.

The inventors have found that a certain ferroelectric material has thedirection of its spontaneous polarization reversed when a fieldexceeding the magnitude of the threshold field peculiar to the material(hereinafter referred to as coercive field) or a stress exceeding themagnitude of the threshold stress peculiar to that material (hereinafterreferred to as coercive stress) is applied to that material and that thelattice strain produced in the crystallographic aand b-axes of thematerial is equivalent to 90 rotation of the material about thecrystallographic c-axis thereof and this phenomenon contains therein amemorizing action.

Years of research carried out by the inventor on ferroelectric materialsshow that a certain ferroelectric material such as potassium dehydridephosphate (hereinafter referred to as KDP) or gadolinium molybdate(hereinafter referred to as GMO) has a different property from the knownferroelectric materials such as triglycine sulfate, leadzicornatetitanate, barium titanate, etc. That is, if a field or a stressexceeding the threshold peculiar to such a certain material (hereinafterreferred to as coercive field or coercive stress) is applied to thatmaterial, the direction of its spontaneous polarization 1 is reversed toshift from condition (a) to condition (b) or from condition (b) tocondition (a), as shown in FIG. 1, depending on whether the originaldirection of the spontaneous polarization is positive or negative(downward or upward as viewed in FIG. 1). It has also been found thatthe crystal lattice is deformed as the aforesaid shift takes place andthat this deformation is equivalent to a 90 rotation of the materialabout the crystallographic c-axis thereof. Thus, the inventors havediscovered that such a material whose spontaneous polarization can bereversed by a field or a stress applied thereto and which can provide acrystal deformation falls within certain point groups of ferroelectricmaterials. More specifically, such a property is owned by certainmaterials belonging to point groups mm2, 2-1 and 2-H. These materialsare classified into point groups imm2, i2-I and i2-II, where i meansirregular ferroelectrics, m represents the mirror symmetry inF-operation, 2I represents a material of crystal lattice having atwo-fold symmetry axis and whose 90 rotation about this axis enables thecrystal lattice to register with the original crystal lattice of thatmaterial, and 2-H represents a material of crystal lattice having atwofold symmetry axis and whose 180 rotation about this axis enables thecrystal lattice to register with the original crystal lattice. Materialsrepresented by imm2, i2-I, and i2-II also include irregularferroelectric materials belonging to these point groups.

Any of the foregoing irregular ferroelectric materials shows ahysteresis loop as shown in FIG. 2a between an applied field E and thepolarization P resulting therefrom, a hysteresis loop as shown in FIG.3a between an applied field E and the strain X resulting therefrom, ahysteresis loop as shown in FIG. 3a between an applied stress and thepolarization resulting therefrom, and a hysteresis loop as shown in FIG.3b between an applied stress and the strain resulting therefrom. Inthese graphs, E X P,,, and X, represent coercive field, coercive stress,spontaneous polarization, and spontaneous strain respectively. As willbe apparent from these graphs, it has been found that the aforementionedirregular ferroelectric materials have a ferroelectric property as wellas a property which may be called a ferroelastic property. Thus, suchmaterials are hereinafter referred to as ferroelectric-ferroelasticmaterials, and they include the materials shown in the table below.

Ammonium aluminum methyl sulfate l,2-hydride Any of such crystals showsan optically biaxial birefringence for the ferroelectric phase thereof,and has different refractive indices a, B, and y for light vibratingparallel to the optically elastic axes X, Y, and Z of the crystal. Forexample, in case of a GMO single crystal which belongs to the pointgroup mm2, n,.=l.8428, n3=l.8432 and Ily=l.897 at room temperature, for)t 589.3. From this it is apparent that a crystal belonging to the pointgroup mm2 shows a double refractivity as a biaxial crystal. Assume thatsuch a GMO crystal is Z-cut (i.e., the end faces of the crystal are cutaway perpendicularly with respect to the crystallographic cards) andinterposed between the olarizer 2 and analyzer 3 with their vibrationplanes intersecting each other but with their polarization planesdisposed parallel to each other, in such a manner that the Z-axis of thecrystal is disposed perpendicularly with respect to the polarizationplanes of the polarizer and analyzer, as shown in F l6. 4. If naturallight enters the polarizer 2 perpendicularly thereto, the natural lightis formed into linearly polarized light by the polarizer 2 and isfurther formed into elliptically polarized light by the birefringence ofthe crystal plate 5. Further, only the component of the circularlypolarized light which is equal to the polarization plane of the analyzer3 is allowed to pass through the analyzer 3, whereby an interferencecolor can be observed due to its retardation with respect to the lightsof various wavelengths forming natural light.

Assume that one of these irregular ferroelectric crystals which has theimm2 symmetry is cut into a cube whose edges are provided by the crystalaxes of the crystal and whose faces are polished into flat opticalplanes, that a transparent electrode is provided on the Z-surface of thecrystal (i.e., that surface which is cut normal to the Z-axis of thecrystal), and that this crystal is interposed in a diagonal position(slightly inclined) between two crossed-Nicol prisms. lf natural lightenters such a system, there appear interference colors provided bybirefringence in accordance with the thickness of the crystal. Thisresults from the phase difference or retardation R between the lightbeams caused by the crystal, and this retardation, as is well-known, hasthe following relation to the thickness d of the crystal through whichlight passes and the difference An between the two refractive indices ofthe double refraction:

R And In an irregular ferroelectric material, as described above, thecrystallographic aand b-axes are interchanged by spontaneouspolarization reversal, and this in turn causes a variation both in thethickness d and the refractive index difference An with respect to thelight. If R(+) and R(--) represent the retardations for the lightentering in the directions perpendicular to the polarization axescorresponding to the positive and negative polarizations, for example,in the directions of X and Y (which retardations are referred to as atransverse effect), then the following relations will be derived:

RH Since normally (d,d,,)/(d, d,,) 0.01 to 0.001 and (n 11,, )llly :1 to0.1 the polarization reversal results in a variationin theiriterferencecolor provided by birefringence. Thus there can be obtained an elementwhich can vary vivid colors with each other in accordance with thethickness thereof.

However, in case of the foregoing GMO single crystal,

llg ll /lljv 2 X10, hence, ll -ll llly ill This means a disadvantage fora GMO single crystal in that its color variation range is too narrow toprovide a color variation by utilizing the retardation variationresulting only from the variation in the birefringence of a transverseeffect caused by polarization reversal.

The present invention provides a system different from theabove-described one, namely, a novel system in which the direction ofpolarization is in accord with the direction of transmitted light.

Therefore, according to the present invention, both the length of thelight path d and the birefringence n l n I are made invariable by thepolarization reversal, the sign of the birefringence is changed, andaccordingly, a number of unique phenomena take place in thedouble-refracted light passing through such an element. Description willnow be made of such phenomena and the principle underlying them.

Applying an electric field exceeding the coercive field to the Z-cut GMOcrystal plate 3 causes the reversal of the spontaneous polarizationthereof, which in turn causes the optic axial plane of the crystal torotate whereby the direction of the elliptically polarized lightresulting from the retardation between the double-refracted lighttransmitted through the crystal is reversed. Thus, the retardation R inthis case is equal to that having the opposite sign.

If a double-refractive transparent crystal 6 and a Z-cut irregularferroelectric crystal 5 such as a Z-cut GMO crystal are disposed in apolarizing system comprising a polarizer 2 and an analyzer 3 opposed inparallel to each other, in such a manner that the Z-cut surface of theZ-cut crystal 5 is perpendicular to the optical axis of the polarizingsystem and that the principal axes of the two crystals are in thedirection, as shown in FIG. 4, then the value of the resultingretardation will be the sum of or the difference between retardations Rand R in accordance with the direction of the optic axial plane of theGMO crystal.

Also, if two Z-cut single crystals belonging to the point group imm2 andhaving retardations R and R respectively and a double-refractive crystalhaving a retardation R are disposed in such a manner that the Z-cutsurfaces thereof are perpendicular to the incident linearly polarizedlight and the other two axes are aligned with each other, and if anelectric field is applied to the two single crystals belonging to thepoint group imm2 to suitably reverse their spontaneous polarization,then there is established a summing or subtracting relationship betweenR R and R provided by the respective crystals. A summing relationshiptakes place when the crystal polarization is not reversed, whereas whenthe polarization is reversed by a field applied to the GMO singlecrystals a subtracting relationship occurs so that the resultingretardation of the two biaxial crystals is represented by R R Thus, theinterference colors provided by the various wavelengths of the naturallight incident upon the polarizing system are varied for R R and R RAccording to the present invention, it is further possible to interposea combination of one or more double-refractive crystal plates and one ormore irregular ferroelectric crystal plates belonging to the point groupimm2 between the parallel polarizer and analyzer, so that an electricfield is applied to the irregular ferroelectric crystals to reverse thespontaneous polarization thereof to thereby provide a kind of opticalshutter device.

According to the present invention, shutter action is effected fornatural light by selecting the thickness of the respective crystals sothat the resulting retardation of the double-refracted light transmittedthrough the double-refractive crystal and the irregular ferroelectriccrystal is within a predetermined range.

The above-described optical shutter device of the present inventionenables the quantity of light passing through a thin plate of colorlesstransparent crystal to be readily controlled by a voltage, and thisleads to the following advantages:

1. electrically controllable arrangement;

2. no extraneous force is required to maintain the stop condition;

3. very high response speed which is in order of 1/1000 second;

4. simple construction of the device, thus economizing the spacerequired;

5. a high shock-resistivity; and

6. semi-permanent life with ease of maintenance of the device. Thus, thepresent invention has a great industrial value in that it is effectivelyapplicable to the stop in an EE camera, the automatically operated stopin a television telephone camera, or the digital pattern producingelement for recording information in the optical memory of an electroniccomputing machine by the use of laser holography or like technique.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention will now bedescribed by way of exam le in conjunction with the accompanyingdrawings.

Referring to FIGS. 5a and 5b the optical shutter device of the presentinvention comprises a polarizer 2 having the vibration plane thereofdisposed along the y-axis, an analyzer 3 having the vibration planethereof disposed along the x-axis, a first ferroelectric-ferroelasticdouble-refractive crystal plate 5 whose optical axes X and Y are adaptedto rotate 90 about the Z-axis, and a second double-refractive crystalplate 6, both crystal plates being interposed between the polarizer 2and analyzer 3. While FIGS. 50 and 5b show one crystal plate 5 and onecrystal plate 6, the number of these crystal plates may be greater asdesired. The arrangement is such that applying a voltage or a stress tothe first crystal plate 5 causes this crystal to rotate the optic axialplane thereof by 90 about the Z-axis and that the resultant R of theretardations provided by the first and second crystal plates 5 and 6 canshift between condi tion I (R and condition II (80 2 R 2 320) or betweencondition I and condition Ill (2000 E R).

FIG. 6 shows the relation between the optical axes X, Y, Z and spatialaxes x, y, z of the crystals, in which axes Z and z are in accord withthe direction of light path and axes X and Y are inclined clockwise byan angle 0 with respect to the xand yaxes,respectively.

With the above-described arrangement, the linearly polarized lightprovided through the polarizer 2 passes through the first and seconddouble-refractive crystal plates and 6 so as to be formed intoelliptically polarized light. More concretely, condition I, i.e., R 0takes place in the closed position of the shutter device so that thelinearly polarized light provided through the polarizer 2 remains as itis, and stopped by the analyzer 3. On the other hand, in the openposition of the shutter, condition II, i.e., 80 5 R 320 or conditionIll, i.e., 2000 E R takes place so that there is provided ellipticallypolarized light whose greater component passes through the analyzer 3,and thus most of the light is allowed to pass through the analyzer 3 toeffect a shutter action. Alternatively, it is possible to omit theanalyzer 3 and provide a mere element which can rotate the vibrationplane of the linearly polarized light by 90", instead of acting as ashutter element.

lfthe retardations provided by the first and second doublerefractivecrystal plates are selected otherwise, then it is of course possible touse the shutter device of the present invention not only with naturallight but also with monochromatic light.

Description will be made of the manner in which the shutter action iseffected by utilizing the fact that the linearly polarized lightprovided through the polarizer 2 is usually formed into ellipticallypolarized light through the doublerefractive crystal plate 5.

A wavelength A is taken as an example. The linearly polarized lightprovided through the polarizer 2 is expressed as y=A sin 2'n't/T (1) Ifthe thickness of the crystal plate 5 is d and the difference between thetwo different refractive indices of the doublerefracted light for thewavelength A is n n,, then the retardation 8 for light of wavelength Ais given thus:

6=2rrd(n,n,)/A (2) Hence, components 2: and y of the amplitude of theelliptically polarized light after passing through the crystal plate 5are given as follows:

A =A sin 28 sin 6/2 A, A (cos 26 sin 2t9cos 8/2) Thus, if the lightpassing through the analyzer 3 is adjusted to be zero or to assume acertain value, there is then provided an optical shutter.

In other words, if it is possible to establish the following relations:

a. Ax =t= O for any wavelength in the On position of the shutter, and b.Ax 0 for any wavelength in the Off position of the shutter,

then the optical shutter is effective for natural light. Now that 8 isthe only term related to A as seen from the equation (3), the condition(b) above can be satisfied if 6 could be 0 independently of wavelength.The condition (a) above is also possible, because if A is limited withinthe wavelength range of visible light the following relation can bechosen:

8/2 0, 11, Zn (for any A) The foregoing example refers to the case wherecrossed- Nicol prisms are used. Discussion will now be made for the caseof parallel-Nicol prisms. In order to provide an optical shutter fornatural light, the same conditions as shown above must be met, that is,

a. Ay =t= 0 for any wavelength in the On" position of the shutter, and

b. Ay 0 for any wavelength in the Off position of the shutter. Assumethat A is limited within the wavelength range of visible light. Then,the condition (a) above can be satisfied by the equation (4), whereasthe condition (b) cannot be satisfied unless, when 26= 8/2 =1r/2, 31-r/2(for any A) However, this relation is usually incompatible with theequation (2) which expresses 8.

For any given wavelength, parallel-polarizers provide a shutter fornatural light because either Ay 0 or Ay A is possible.

The case of cross-polarizers will be further discussed with respect tothe aforementioned conditions (a) and (b). In the condition (b), therelation that Ax 0 for any wavelength can be simply attained byobtaining the relation that 6 0, i,e., by nullifying thedouble-refraction effect. This can be accomplished, for example, byarranging two identical crystals with their optical axes disposed insuch directions that their respective double,refractions negate oneother. As regards the condition (a), it is desirable to attain therelation that Ax O for any wavelength but attain the relation that A): Afor the wavelength at a given point so that as much visible light aspossible may pass through the crossed-polarizers. If the analyzer usedsubsequently for the shutter has a flat wavelength sensingcharacteristic as shown in FIG. 7a, a wavelength )tmax providing Ax Amay be adjusted to the mean wavelength km of the visible light, tothereby provide the characteristic as shown in FIG. 4a. If thewavelength sensing characteristic of the analyzer is such that thewavelength sensitivity thereof reaches the peak value for a specificwavelength M as shown in FIG. 7b, and also if M )rm, then the analyzeris adjusted to show the characteristic as shown in FIG. 8b for Amax )tm.In case of the characteristic as shown in FIG. 7c, the analyzer isadjusted to show the characteristic of FIG. 8c so as to provide therelation that kmax M.

In addition to the examples of the transmission characteristic having asingle peak value in the "On" position of the shutter as shown in FIGS.80, 8b, and 8c, use may be made of the characteristic having a pluralityof peak values as shown in FIG. 9 to obtain transmitted lightapproximately equal to natural light.

This will now be described with reference to a CIE chromaticity chart.The retardation provided by the crystal is defined by the followingequation:

R=2 1r/Ad(n -n where d represents the thickness of the crystal plate 5and n n represents the birefringence for the wavelength A. If thethickness d of the crystal plate 5 is varied, the retardation R is alsovaried. FIG. 10 shows a CIE chromaticity chart illustrating the mannerin which color changes for various retardation values. In the chart, theangular coordinates x and y represent .1: and y of the three primarycolors, the outer curve (indicated by a solid line) represents the locusprovided by a series of spots showing the spectral colors, and thehorseshoeshaped area defined by the said locus and the straightdashand-dot line AB Qcorresponding to pure violet) indicates variousactual colors. The numerical values given in FIG. 10 show the variousvalues of R. The chromaticity of a light source having a luminousspectrum (i.e., natural source of light) lies at the coordinates:

From this position toward the circumference of the horseshoe shape, thedegree of color mixture decreases to approach pure colors provided bythe respective spectra. As seen from FIG. 10, if R 0, there is notransmitted light, and in the range of R 80 to 320, substantially whitecolor is provided. As R increases to exceed 2,000, there is againprovided substantially white color. This is because the spectrum oftransmitted light becomes as shown in FIG. 9.

Accordingly, if the conditions I, II, and III are defined thus:

Condition 1: R 0

Condition II: 80 R 320 Condition III: 2000 R then the shutter actionwill be such that the shift of condition can take place either fromcondition I to condition II or III or conversely, that is, either fromright to left or from left to ri ht.

Examples of the present invention will be shown below.

EXAMPLE 1 As shown in FIG. 11, two first and second double-refractiveZcut GMO single crystal plates 5 and 6 are disposed in a diagonalposition between a polarizer 2 and an analyzer 3.

The two polarizers are arranged to provide crossed-fields ofpolarization. Each of the Z-cut single crystal plates 5 and 6 has athickness of 330p. If the Z-cut single crystal plates 5 and 6 aredisposed with their axes X and Y deviated 90 from each other, that is,in such a manner that the axes X and Y of the zcut single crystal plate5 are in accord with the axes Y and X of the other Z-cut single crystalplate 6, then the retardation provided by these Z-cut single crystalplates 5 and 6 is R R R 0 R R because they have the same thickness, andthus there is no light other than the leakage light resulting from thecrossed-polarizers.

If the X- and Y-axes of the single crystal plate 5 is interchanged by afield (operable at 150 volts) applied thereto, the X- and Y-axes of bothcrystal plates are in accord with each other to provide a summingrelation between their retardations, hence In case of Z-cut GMO singlecrystal plates having a thickness of 330 u,

R, R 138 my and therefore the spectrum transmitted through thepolarizing unit for R 275 mg. is as shown in FIG. 12. T 100 percent for550 mp. and thus, for A 400 my. to 760 mp, the energy transmittivity asa whole is 92 percent.

When an experiment has been carried out by using two commerciallyavailable polarizers, the ratio of quantity of light stopped to thetotal quantity of light transmitted has proved to be 121000, i.e., 60dB, which means a ratio 1:920 for the aforesaid crystal plates 5 and 6.This provides a practically sufficient damping ratio.

The crystal plate 5 may be any plate providing a retardation of 138 my,namely, having a wavelength corresponding to the mean wavelength ofnatural light, and thus quartz, for instance, may be used.

When use is made of an X-cut or Y-cut quartz plate, the thicknessthereof is l6p.. The quartz plate is disposed in a diagonal position.

EXAMPLE 2 Referring to FIG. 13, four crystal plates 5, 6, 7, and 8 aredisposed between the polarizer 2 and analyzer 3.

The crystal plate 5 is a Z-cut GMO crystal having a thickness of 33011..The optical axis Y of the crystal plate 5 is clockwise inclined by anangle 0 with respect to the vibration plane of the polarizer 2. Thecrystal plate 6 may be any material having a retardation of 138 mu, butthe optical axes thereof must be perfectly in accord with those of thecrystal plate 5. Thus, light having a wavelength A 550 mp. passesthrough the set of crystal plates 5 and 6 to be thereby rotated 20 withrespect to the vibration plane of the polarizer 2. The crystal plate 7is formed of Z-cut GMO crystal and disposed with the optical axis Ythereof inclined by an angle a counterclockwise with respect to thevibration plane of the polarizer 2.

The crystal plate 8 may be formed of any material having a retardationof 138 mp, such as Z-cut GMO plate with a thickness of 330g. The opticalaxes of the crystal plate 8 are perfectly in accord with those of thecrystal plate 7. Thus, light having a wavelength l\ 550 mp. passesthrough the set of crystal plates 7 and 8 to thereby rotate a further201. After all, the light is rotated an angle of (26 20:) in total withrespect to the vibration plane of the polarizer 2. Therefore, if 0 a 45,light having a wavelength of 550 mp. is formed into linearly polarizedlight whose polarizafion plane has been rotated by and percent passes ofit through the analyzer 3.

The advantage of this arrangement is that the sets of crystal plates 5,6 and 7, 8 may be combined into a block so as to provide 0 a 45, wherebythe orientation of the block and that of the polarizer can be freely setto suitable angles as desired.

In practice, such orientations may be adjusted and fixed to certainangles suitable for passing the greatest possible quantity of lightafter the assemblage has been completed.

When the shutter is to be closed, a voltage is applied to the GMOcrystal plates 5 and 7 to rotate the optical axes X and Y by 90 so as toprovide zero retardation in the respective sets of crystal plates 5, 6and 7, 8, thus resulting in crossed polarities of the polarizer 2 andanalyzer 3, and accordingly in no transmitted light.

For the clarity of description, the foregoing two examples have beenillustrated with respect to the case where only a voltage is applied foroperation, whereas a stress may be applied for control, if desired, aswill be shown in the following example.

EXAMPLE 3 As shown in FIGS. 11 and 13, Z-cut GMO single crystal plateseach having a thickness of 330;; are used to form a polarizing unit,which is additionally provided with press means as shown in FIGS. 14aand 14b.

The press means shown in FIGS. 14a and 14b is directed to operate thedouble-refractive crystal plates, an element 9 formed with facesperpendicular to the axes X and Y of the crystal plate and facesparallel to the Z-axis of the crystal, and a Z-cut disc 10. The pressmeans comprises fixing elements 11, and pressing elements 12 and 13having projections 14 and 15 supported by guides 16.'The press means isdesigned such that the pressure exerted on the projections 14 and 15 ofthe pressing elements 12 and 13 imparts a pressure normal to the X- andY-axes of the crystal plates.

If one of the projections 14 and 15 of the pressing elements 12 and 13,e.g., the projection 15, is pressed to apply a pressure to the crystalplate along the Y-axis thereof, the crystal will shrink so that thecrystallographic Y-axis thereof turns into the X-axis, which means a 90rotation of the optical axis. However, when the crystallographic Y-axishas turned into the X axis, any more pressure applied to the crystalplate would cause no further variation therein.

Thus, the crystallographic Y-axis changes into a position rotated 90with respect to the Z-axis. This position is in accord with thedirection in which the projection 14 of the other pressing element 13 ispressed, and therefore, if the projection 15 of the pressing element 15is again pressed, the optic axial plane of the crystal is again rotated90 with respect to the Z- axis to return to its original position.

By pressing the orthogonal pressing elements alternately in theabove-described manner, the optical axis can be rotated 90 and returnedto its original position. The polarizing unit of FIG. 11 or FIG. 13provided with such elements can provide a shutter device for naturallight which can stop or pass the light by the use ofan applied stress.

EXAMPLE 4 Two Z-cut GMO crystal plates each having a thickness of3,00011. are prepared. One of the two GMO crystal plates is providedwith a transparent electrode formed ofSnO on each Z-cut face thereofover the entire area. The electrodes are connected to a desired voltagesource. Both the GMO crystal plates are put in a state ofsingle domainand put together such that their azimuths are in agreement with eachother. Then, the composite retardation between the double-refractedlight passed through the composite GMO crystal plate is 2,500 mp When anelement consisting of the composite GMO crystal plate is interposed indiagonal relationship between a pair of parallel polarizer platesarranged such that their polarizing directions are orthogonal, a desiredoptical shutter device is provided. In this device, ifa voltage isapplied to the one GMO crystal plate provided with the electrodes toreverse the polarization of the crystal plate, the sign of retardationtherein becomes opposite to that in the other crystal plate. Since thethickness of the two plates are equal to each other, the light linearlypolarized by the polarizer on the entrance side passes through theelement without being affected thereby, but is shut out by the polarizeron the exit side. When the signs of the retardations of the two GMOcrystal plates are equal, the retardations are added together to becomea total of 2,500 my.

For the above-described operation to be realized, the fact that thethicknesses of the two crystal plates are equal to each other is anecessary condition. For the total retardation R 2,000 my or more, it issufficient for the thickness of each GMO crystal to be 2,400,u. or more.

While the above example has been described with respect to the casewhere the crystal plate is formed of GMO in order to explain theprinciple of the optical shutter according to the present invention, usemay also be made of any material having ferroelasticity. In addition toGMO and isomorphic materials, it is possible to use, for example,potassium hydrogen phosphate (KDP), boracite or the like, although thefollowing requirements must be met. KDP must be used after being dippedin liquid nitrogen, for instance, since it must be kept at a temperatureof l50 C or below. Also, boracite or like material must be formed intoabout one tenth of the thickness of GMO, say, 34p, since the value oftheir birefringence are about 10 times larger than that of GMO.

What is claimed is:

1. An optical shutter comprising a pair of crossed polarizing means, atleastone Z-cut irregular ferroelastic-ferroelectric crystal plate and atleast one double-refractive crystal plate both interposed between saidpair of polarizing means, said crystal plates having a thicknessselected so that the retardation of double-refractive light passingthrough said doublerefractive crystal plate is equal to the retardationof doublerefractive light passing through said Z-cut irregular ferroelastic-ferroelectric crystal plate and that the value of theresulting retardation R of said crystal plates in the open position ofsaid shutter satisfies one of the relationships S R S 320, 2,000 S R,and means provided on said irregular ferroelastic-ferroelectric crystalplate for applying thereto a field or stress at least equal to themagnitude of the coercive field or stress, respectively, of that crystalplate.

2. An optical shutter as defined in claim 1, wherein said irregularierroelastic-ferroelectric crystal is gadolinium molybdate.

3. An optical shutter comprising a pair of crossed polarizing means, atleast one Z-cut irregular ferroelectric crystal plate and at least oneplate of double-refractive material plate both interposed between saidpair of polarizing means, said crystal plates having a thicknessselected so that the retardation of double-refracted light passingthrough said plate of doublerefractive material is equal to theretardation of doublerefracted light passing through said Z-cutirregular ferroelectric crystal plate and that the value ofthe resultingretardation R of said crystal plates in the open position of saidshutter becomes 8O 2 R 320, a transparent electrode provided in theZ-cut plane of said irregular ferroelectric crystal, and electric meansfor applying through said transparent electrode a field at least equalto the magnitude of the coercive field of said irregular ferroelectriccrystal.

4. An optical shutter as defined in claim 3, wherein said irregularferroelectric crystal is gadolinium molybdate.

5. An optical shutter as defined in claim 4, wherein said transparentelectrode is formed of SnO 6. An optical shutter comprising a pair ofcrossed polarizing means, at least one Z-cut irregular ferroelectriccrystal plate and at least one double-refractive material plate bothinterposed between said pair of polarizing means, said crystal plateshaving a thickness selected so that the retardation of double-refractedlight passing through said double-refractive material plate is equal tothe retardation of double-refracted light passing through said Z-cutirregular ferroelectric crystal plate and that the value of theresulting retardation R of said crystal plates in the open position ofsaid shutter becomes R 2 2000, a transparent electrode provided in theZ-cut plane of said irregular ferroelectric crystal, and electric meansfor applying through said transparent electrode a field at least equalto the magnitude of the coercive field of said irregular ferroelectriccrystal.

7. An optical shutter as defined in claim 6, wherein said irregularferroelectric crystal is gadolinium molybdate.

8. An optical shutter as defined in claim 7, wherein said transparentelectrode is formed of SnO or lnO 9. An optical shutter comprising apair of crossed polarizing means, at least one Z-cut irregularferroelectric crystal plate and at least one double-refractive crystalplate both interosed between said pair of polarizing means, said crystalplates having a thickness selected such that the retardation ofdouble-refracted light passing through said double-refractive crystalplate is equal to the retardation of the double-refracted light passingthrough said Z-cut irregular ferroelectric crystal plate and that thevalue of the resulting retardation R of said crystal plates in the openposition of said shutter becomes 80; R g 320, and pressure means areprovided to said irregular ferroelectric crystal plate for applyingthereto a stress at least equal in magnitude to the coercive stress ofthat crystal plate by means of pressing elements movable along the X-and Y- axes of said crystal plate.

10. An optical shutter as defined in claim 9, wherein said irregularferroelectric crystal is gadolinium molybdate.

ll. An optical shutter comprising a pair of crossed polarizing means, atleast one Z-cut irregular ferroelectric crystal plate and at least onedouble-refractive crystal plate both interposed between said pair ofpolarizing means, said crystal plates having a thickness selected sothat the retardation of double-refracted light passing through saiddouble-refractive crystal plate is equal to the retardation ofdouble-refracted light passing through said Z-cut irregularferroelectric crystal plate and that the value of the resultingretardation R of said crystal plates in the open position of saidshutter becomes R 2 2000, and pressure means provided to said irregularferroelectric crystal plate for applying thereto a stress at least equalto the magnitude of the coercive stress of that crystal plate by meansof pressing elements movable along the X- and Y-axes of said crystalplate.

12. An optical shutter as defined in claim 11, wherein said irregularferroelectric crystal is gadolinium molybdate.

1. An optical shutter comprising a pair of crossed polarizing means, atleast one Z-cut irregular ferroelastic-ferroelectric crystal plate andat least one double-refractive crystal plate both interposed betweensaid pair of polarizing means, said crystal plates having a thicknessselected so that the retardation of double-refractive light passingthrough said double-refractive crystal plate is equal to the retardationof double-refractive light passing through said Z-cut irregularferroelastic-ferroelectric crystal plate and that the value of theresulting retardation R of said crystal plates in the open position ofsaid shutter satisfies one of the relationships 80 < OR = R < OR = 320,2,000 < OR = R, and means provided on said irregularferroelastic-ferroelectric crystal plate for applying thereto a field orstress at least equal to the magnitude of the coercive field or stress,respectively, of that crystal plate.
 2. An optical shutter as defined inclaim 1, wherein said irregular ferroelastic-ferroelectric crystal isgadolinium molybdate.
 3. An optical shutter comprising a pair of crossedpolarizing means, at least one Z-cut irregular ferroelectric crystalplate and at least one plate of double-refractive material plate bothinterposed between said pair of polarizing means, said crystal plateshaving a thickness selected so that the retardation of double-refractedlight passing through said plate of double-refractive material is equalto the retardation of double-refracted light passing through said Z-cutirregular ferroelectric crystal plate and that the value of theresulting retardation R of said crystal plates in the open position ofsaid shutter becomes 80 < or = R < or = 320, a transparent electrodeprovided in the Z-cut plane of said irregular ferroelectric crystal, andelectric means for applying through said transparent electrode a fieldat least equal to the magnitude of the coercive field of said irregularferroelectric crystal.
 4. An optical shutter as defined in claim 3,wherein said irregular ferroelectric crystal is gadolinium molybdate. 5.An optical shutter as defined in claim 4, wherein said transparentelectrode is formed of SnO2.
 6. An optical shutter comprising a pair ofcrossed polarizing means, at least one Z-cut irregular ferroelectriccrystal plate and at least one double-refractive material plate bothinterposed between said pair of polariZing means, said crystal plateshaving a thickness selected so that the retardation of double-refractedlight passing through said double-refractive material plate is equal tothe retardation of double-refracted light passing through said Z-cutirregular ferroelectric crystal plate and that the value of theresulting retardation R of said crystal plates in the open position ofsaid shutter becomes R > or = 2000, a transparent electrode provided inthe Z-cut plane of said irregular ferroelectric crystal, and electricmeans for applying through said transparent electrode a field at leastequal to the magnitude of the coercive field of said irregularferroelectric crystal.
 7. An optical shutter as defined in claim 6,wherein said irregular ferroelectric crystal is gadolinium molybdate. 8.An optical shutter as defined in claim 7, wherein said transparentelectrode is formed of SnO2 or InO2.
 9. An optical shutter comprising apair of crossed polarizing means, at least one Z-cut irregularferroelectric crystal plate and at least one double-refractive crystalplate both interposed between said pair of polarizing means, saidcrystal plates having a thickness selected such that the retardation ofdouble-refracted light passing through said double-refractive crystalplate is equal to the retardation of the double-refracted light passingthrough said Z-cut irregular ferroelectric crystal plate and that thevalue of the resulting retardation R of said crystal plates in the openposition of said shutter becomes 80 < or = R < or = 320, and pressuremeans are provided to said irregular ferroelectric crystal plate forapplying thereto a stress at least equal in magnitude to the coercivestress of that crystal plate by means of pressing elements movable alongthe X- and Y-axes of said crystal plate.
 10. An optical shutter asdefined in claim 9, wherein said irregular ferroelectric crystal isgadolinium molybdate.
 11. An optical shutter comprising a pair ofcrossed polarizing means, at least one Z-cut irregular ferroelectriccrystal plate and at least one double-refractive crystal plate bothinterposed between said pair of polarizing means, said crystal plateshaving a thickness selected so that the retardation of double-refractedlight passing through said double-refractive crystal plate is equal tothe retardation of double-refracted light passing through said Z-cutirregular ferroelectric crystal plate and that the value of theresulting retardation R of said crystal plates in the open position ofsaid shutter becomes R > or = 2000, and pressure means provided to saidirregular ferroelectric crystal plate for applying thereto a stress atleast equal to the magnitude of the coercive stress of that crystalplate by means of pressing elements movable along the X- and Y-axes ofsaid crystal plate.
 12. An optical shutter as defined in claim 11,wherein said irregular ferroelectric crystal is gadolinium molybdate.